In recent years, an electronic paper display device that is a display device with a memory capability has been developed as a display device such that which “reading” can be realized with no stress such as an electronic book, an electronic newspaper or the like. Here, the “display device with a memory capability” means a display device which has two or more different stable states and of which the two or more different stable states can be electrically switched and those states have different optical properties. Such display device is also called as “bistable display device”. The image display state in the bistable display device includes an image update state in which an update image is written and an image non-update state in which an update image is not written. Here, because the bistable display device can hold the image displayed on the display device without supplying electrical power when it is in the image non-update state, the power consumption thereof can be reduced.
In particular, the above-mentioned electronic paper display device has a memory capability that is the feature of the bistable display device mentioned above and additionally, it is requested to be thin, lightweight, resistant to cracking and easy to read like a printed paper. Additionally and preferably, it is desired that the display panel of the electronic paper display device is flexible (bendable) and can perform a color display. As an example of the display device with a memory capability, the microcapsule type electrophoresis display device developed by an American company (E Ink Corporation) is known.
With respect to such electrophoresis display device, a structure in which a microcapsule type electrophoresis display element is driven by a switching element is disclosed in a patent document, for example, U.S. patent application number 2005/0078099 (hereinafter, referred to as patent document 1). The electrophoresis display device disclosed in patent document 1 includes a flexible base substrate, a nonlinear switching element, a pixel electrode connected to the switching element, a microcapsule type electrophoresis display element and a common electrode.
Here, the flexible base substrate is composed of materials such as a stainless steel or a metal foil, polyimide and the like and can be bent. The nonlinear switching element is provided on one surface side of the above-mentioned base substrate. The pixel electrode is connected to the switching element via a planarized film. The microcapsule type electrophoresis display element is arranged on one surface side of the base substrate. The common electrode is provided so as to face the pixel electrode via the microcapsule type electrophoresis display element.
A method for forming the nonlinear element on the flexible substrate in such panel structure is disclosed in for example, Japanese Patent Application Laid-Open No. 2004-219551 (hereinafter, referred to as patent document 2) and Japanese Patent Application Laid-Open No. 2005-292420 (hereinafter, referred to as patent document 3). A method with which the nonlinear element is formed on one surface side of a glass layer, another surface side (rear side) of the glass layer is thinly etched so that the thickness of the glass layer becomes smaller than or equal to 200 μm and a flexible film is pasted on the another surface side of the etched glass layer is disclosed in patent document 2.
A method with which an optical element and a thin film transistor device are transferred and stacked on a long thin film made of an organic resin by a roll to roll process is disclosed in patent document 3.
A plastic substrate such as polyimide, polyethylene terephthalate (PET) or the like other than the stainless steel and the metal foil mentioned above can be used for the flexible substrate disclosed in patent document 1 to 3. Here, the plastic substrate has characteristics in which charge-up (charging) of the surface easily occurs by static electricity or the like in comparison with a conductive substrate such as the stainless steel, the metal foil or the like.
Therefore, it is known that in the display device with a memory capability like the electronic paper display device, when static electricity is generated on the surface of the plastic substrate at the time of image non-update state mentioned above, the electric field induced by this static electricity causes disturbance of an image or deterioration in image quality. That is because an electrophoresis particle moves in a microcapsule by the electric field induced by the static electricity and the image changes from the image in a state in which a display operation has been just finished (at the start of the image non-update state).
A method for preventing charge up of the surface of the plastic substrate is disclosed in for example, Japanese Patent Application Laid-Open No. 2005-208348 (hereinafter, referred to as patent document 4). A structure in which in the electronic paper display device, a transparent electrode film (a first conductive film) formed on a first substrate is electrically connected to an electrode film (a second conductive film) formed on a second substrate that is opposed to the first substrate via the microcapsule is disclosed in patent document 4. Here, the first substrate is provided on a side on which information is displayed by a display element (that is, a viewing side). By using such structure, the charge up of the surface of the plastic substrate used for the first and second substrates can be prevented, and the problem of disturbance of an image or deterioration in image quality at the time of the image non-update state can be solved.